contrasting patterns of mussel abundance at neighbouring sites: does recruitment limitation explain...

312 (2004) 285–298

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Contrasting patterns of mussel abundance at

neighbouring sites: does recruitment limitation

explain the absence of mussels on Cook Strait

(New Zealand) shores?

Jeremy G. Helson*, Jonathan P.A. Gardner

Island Bay Marine Laboratory, School of Biological Sciences, Victoria University of Wellington,

PO Box 600, Wellington, New Zealand

Received 11 July 2003; received in revised form 10 February 2004; accepted 8 July 2004

Abstract

Wellington Harbour (New Zealand) supports large populations of mussels (Aulacomya maoriana,

Mytilus galloprovincialis and Perna canaliculus), whereas these species are absent from Cook Strait

shores only a few km away. The density of planktonic mussel larvae and their recruitment rates to

artificial substrates were investigated at harbour (with mussels) and Cook Strait (no mussels) sites to

determine if a diminished or a zero larval supply and/or settlement explains the absence of mussels

from Cook Strait shores. At both locations, larvae were collected from the plankton approximately

monthly between September 1998 and February 2000, and recruitment rates to artificial substrates

were estimated between March 2000 and February 2001. Planktonic larval densities were almost an

order of magnitude greater within the harbour than at coastal sites (mean (FS.D.) density was 982

m�3 (F1478) with a peak density in September 1998 of 4207 m�3, compared with 106 (F94) and

381 m�3, respectively, in March 1999). Larval recruitment at harbour sites was also significantly

greater than at coastal sites (mean (FS.D.) recruitment density was 2169 m�2 (F4207) with a peak

of ca. 211,425 m�2 in July 2000, compared with 88 m�2 (F86) and ca. 3700 m�2, respectively, in

February 2001). It has been suggested that bbottom upQ regulation of community structure,

principally via a diet of particulates low in organic matter, is the explanation for the absence of

suspension feeding mussels from Cook Strait sites [Helson, J. G., 2001. An investigation into the

absence of mussels (Perna canaliculus, Aulacomya maoriana and Mytilus galloprovincialis) from

0022-0981/$ -

doi:10.1016/j.

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Journal of Experimental Marine Biology and Ecology

see front matter D 2004 Elsevier B.V. All rights reserved.

jembe.2004.07.006

onding author. Tel.: +64 4 463 5574; fax: +64 4 463 5331.

ress: [email protected] (J.G. Helson).

J.G. Helson, J.P.A. Gardner / J. Exp. Mar. Biol. Ecol. 312 (2004) 285–298286

the South Coast of Wellington, New Zealand. Unpublished PhD thesis, Victoria University of

Wellington, 183 pp.], but given that planktonic larval supply and recruitment rates are much reduced

at coastal sites, these data may also be important in explaining the absence. Whether current levels of

recruitment are sufficient to maintain an adult population is at present unknown and requires further

examination.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Aulacomya maoriana; Mytilus galloprovincialis; Perna canaliculus; Wellington Harbour; Cook

Strait; New Zealand; Mussel; Larvae; Plankton; Settlement; Recruitment

1. Introduction

Traditionally, the principal factors regulating temperate intertidal communities have

been thought to be physical variables such as wave exposure and aspect of the

shore, and biological variables such as competition and predation (Lewis, 1964;

Connell, 1972; Paine, 1974; reviewed by Underwood, 2000). More recently, however,

greater emphasis has been placed on understanding the effect of variation in rates of

larval supply, settlement and recruitment to intertidal populations (e.g., Connell,

1985; Menge, 1991; Petraitis, 1991; McQuaid and Phillips, 2000; Swearer et al.,

2002) and how this variability can sometimes be explained by near-shore processes

operating at various spatial (tens to hundreds of kilometers) and temporal scales

(annual, decadal, El Nino Southern Oscillation (ENSO) events) (e.g., Roughgarden et

al., 1988, 1991; Farrell et al., 1991; Ebert et al., 1994; Wing et al., 1995; Graham

and Largier, 1997; Botsford, 2001). Major outcomes of this research include an

improved understanding of the near-shore oceanographic mechanisms contributing to

spatial and temporal variability of propagule recruitment, and a greater appreciation

of how variation in larval supply (regardless of the underlying cause) and recruitment

limitation can contribute to differences in the composition and functioning of

intertidal communities.

In the present paper, we seek to determine why the endemic ribbed mussel

Aulacomya maoriana, the widely distributed blue mussel Mytilus galloprovincialis,

and the endemic greenshell mussel Perna canaliculus are absent from large stretches

of exposed shoreline of Cook Strait. This body of water is orientated almost north–

south and separates the North and South Islands of New Zealand; curiously, all three

mussel species occur abundantly in nearby Wellington Harbour (Fig. 1). This same

question has been addressed from the standpoint of bottom up community regulation.

The feeding physiology, condition index and mortality responses of these mussels to

Cook Strait water that is characterised by low food quality, are all consistent with the

hypothesis of bottom up community regulation (Gardner, 2000; Gardner and

Thompson, 2001). While the demonstration of multi-species food limitation at these

sites is certainly interesting, it is only of ecological relevance if mussel larvae are

transported to these sites and if they are able to settle here. Thus, it was noted that

recruitment limitation in the forms of larval supply and/or settlement could be the

primary explanation for the absence of mussels from these coastal sites, and must

Fig. 1. Location of sampling sites for plankton tows (Oteranga Bay, Island Bay and Kau Bay) and settlement pads

(Island Bay, Moa Pt, Front Lead and Matiu-Somes Is).

J.G. Helson, J.P.A. Gardner / J. Exp. Mar. Biol. Ecol. 312 (2004) 285–298 287

therefore be examined as part of a larger programme whose purpose is to explain and

understand this unusual phenomenon.

Mussels of the family Mytilidae are broadcast spawners with external fertilisation.

Planktonic larvae have traditionally been thought of as passive particles that are

transported by surface circulation, giving them considerable dispersal potential (review

by Caley et al., 1996). However, more recent work has demonstrated that this is not

necessarily always the case and has therefore questioned the universality of this

assumption (McQuaid and Phillips, 2000; Hellberg et al., 2002; Swearer et al., 2002).

Jeffs et al. (1999) estimate the larval phase of P. canaliculus to be 4–6 weeks in duration,

and we assume that M. galloprovincialis and A. maoriana have planktonic phases of

approximately 4 weeks, consistent with data for other mussel species and assumptions


made in comparable studies (Chipperfield, 1953; Seed, 1976; Seed and Suchanek, 1992;

McQuaid and Phillips, 2000). A 20–30-day larval duration for P. canaliculus, M.

galloprovincialis and A. maoriana would provide them with ample opportunity to

colonise Wellington’s south coast (=Cook Strait) from a number of different source

(parental) populations located within a 200–300-km radius. Even allowing for very limited

larval dispersal (e.g., b5 km pa as reported for the invasive mussel M. galloprovincialis in

South Africa: McQuaid and Phillips, 2000), the expectation would still be that native New

Zealand mussels would have colonised Cook Strait from source populations in Wellington

Harbour or from the inner Marlborough Sounds, on the southern side of Cook Strait where

they occur in great abundance (Fig. 1). The question remains as to why they have not.

Recruitment of benthic marine invertebrates with planktonic larvae is a three-stage

process consisting of larval supply, settlement and metamorphosis, and subsequent

survival and growth in the benthic environment (Harrold et al., 1991). The purpose of the

present paper is to examine the first two of these three stages to address the question of

recruitment limitation as an explanation for the absence of mussels from Cook Strait

shores. We specifically do not test the third component of this three-stage process, that is,

recruitment to the adult population (sensu Connell, 1985), because there is no adult

population to recruit to and because our recruitment periods are of approximately monthly

duration throughout the study. We therefore test two null hypotheses: (1) that there is no

significant difference between harbour and coastal sites in mussel larval densities in the

plankton and (2) that there is no significant difference between harbour and coastal sites in

the recruitment rates of mussel larvae. We evaluate our data for planktonic larval densities

and larval recruitment rates in the context of the absence of mussels from Cook Strait sites.

2. Materials and methods

2.1. Larval densities in the plankton

On 14 occasions between September 1998 and February 2000 plankton tows were

carried out at one site inside the harbour (Kau Bay, n=14) and two sites on the south coast

(Island Bay, n=14 and Oteranga Bay, n=13) at approximately monthly intervals (Fig. 1).

Adverse weather prevented sampling at Oteranga Bay in November 1999. Water column

properties at these locations have been described by Helson (2001). The sampling gear

consisted of a 100-Am plankton net attached to a metal ring (0.7 m diameter), inside which

was attached a General Oceanics model 20307 flow meter. This whole assembly was

housed inside a second metal ring (1.0 m diameter), to which was attached a second

General Oceanics model 20307 flow meter. The second flow meter was housed outside the

mouth of the net to record the volume of water filtered and allow the filtration efficiency,

Ef, to be estimated (Ef=Flow Vol.outside/Flow Vol.inside). The flow efficiency, Ef, is a

measure of the degree to which the net became clogged and was unable to filter the water

effectively (Tranter and Smith, 1968). The outer ring was weighted at the bottom and had a

float attached to the top to stop the gear rotating and the flow meter lifting out of the water.

On each sampling occasion, three replicate tows were made at a depth of 1–2 m at each

site. Because mussel larvae are likely to be evenly distributed throughout a shallow, well-


mixed coastal water column (e.g., McQuaid and Phillips, 2000), we consider that our

sampling is representative of the shallow and well-mixed harbour and coastal situations.

All tows were conducted at a speed ofc2 knots and each was ofc5-min duration. At this

speed, the gear worked best, that is, without putting undue strain on the net, while

maintaining enough speed for the flow meters to operate constantly. Tow volumes were

typically 50–100 m3, but net clogging occasionally occurred at Kau Bay because of high

phytoplankton concentration and consequently reduced the volume of some tows to 10–20

m3. At the completion of each tow, the flow meters were read and the sample was washed

into the cod end of the net and then into a jar. Each plankton sample was fixed using 2%

formalin and stored for up to 4 weeks before counting. Samples were stained using Rose

Bengal to distinguish mussel larvae from sand grains and other inorganic material. In the

laboratory, samples were homogenised by gently shaking and inverting the jar. A series of

five subsamples was taken from each of the three replicates and mussel larvae were

counted under a microscope at 10 times magnification. In most instances, the subsamples

taken were 5 ml, with the exception of some harbour samples where the high abundance of

phytoplankton necessitated 3-ml subsamples. The mean number of mussel larvae from the

subsamples was then extrapolated to determine the total in each sample. Mussel veligers

were distinguished from other molluscs using morphometric characteristics (Booth, 1977;

Redfearn et al., 1986). No attempt was made to distinguish among the larvae of each

species because of (1) the high numbers of larvae present in many samples, (2) the

difficulty in distinguishing among species (Booth, 1977) and (3) the fact that we were

primarily interested in the presence versus absence of mussels as a group.

Asymmetrical analysis of variance (e.g., Underwood, 1997) was used to test for

differences in the density of mussel larvae among sites as a function of time.

Asymmetrical ANOVA was used because we had two Cook Strait sites (Oteranga Bay

and Island Bay) and only one Harbour site (Kau Bay). We tested the effects of locale

(either Cook Strait or Wellington Harbour), site in locale, month and the site�month

interaction term on the density of mussel larvae. All mussel count data were log10transformed prior to testing to satisfy the assumptions of normality and equality of

variances for this parametric test. Comparisons were made among nine months, data were

not collected for the month of July and were incomplete for December and March. On

occasions where months were represented in both years, data were included in the analysis

from both months.

2.2. Larval densities at recruitment

In March 2000, settlement pads were deployed at two sites within Wellington

Harbour (Matiu-Somes Island and the Front Lead) and two coastal sites (Moa Point and

Island Bay) with three replicates per site (Fig. 1). The high energy environment of Cook

Strait required the trial of several different artificial substrates and anchoring systems, as

a result the plankton and recruitment experiments were not conducted simultaneously.

Pads consisted of nylon shade cloth that had been folded five times to form a pad

200�200 mm. Pads were not conditioned before use as this has been shown to be

unnecessary for similar nylon substrates (McGrath et al., 1994). The experiment ran

from March 2000 to February 2001. At both harbour sites, a rope collar was tied around

Fig. 2. Mussel larval density in the plankton at the three study sites (a) Kau Bay, (b) Island Bay, (c) Oteranga Bay,

(d) data from all sites. Points are the mean of three samples and error bars show one standard deviation. Note

logarithmic y-axes in all figures.



a wharf pile and the settlement pads were attached to the rope with cable ties; pads were

c3 m below chart datum. At the coastal sites, 20-l buckets of concrete with steel

reinforcing wire embedded in the top were used as attachment points for the settlement

pads. These buckets were placed in relatively sheltered locations at the two south coast

sites and were at a level that ensured they were always covered with water and at a

comparable depth to pads in the harbour. Pads were changed after c30 days and were

then left to soak for 24 h in a solution of freshwater and 2% formalin (preliminary tests

of various extraction methods indicated that this approach worked best in this

environment). Each pad was cleaned using a moderate flow of freshwater to wash

larvae into a 20-l bucket. This water was filtered (125-Am sieve) and the sieve contents

transferred to a labeled jar where the sample was treated with a 1% solution of formalin

and Rose Bengal to distinguish mussel spat from inorganic material. The contents of

each jar were shaken gently and inverted several times to ensure the sample was

homogeneous, before the number of mussels in each of 5�5-ml subsamples was counted

under a microscope at 10 times magnification. The total number of mussel larvae in each

20-l sample (and therefore per settlement pad) was extrapolated from the five

subsamples.

ANOVAwas used to test for differences in numbers of settled mussels among sites and

times of the year. A separate ANOVAwas conducted to test for differences in recruitment

density between the combined harbour sites and combined coastal sites. Both tests were

performed on log10 transformed data to satisfy the assumptions of normality and equality of

variance.

3. Results

3.1. Larval densities in the plankton

Higher densities of planktonic mussel larvae were observed within the harbour (Kau

Bay) than at the coastal sites (Island Bay or Oteranga Bay) (Fig. 2). Larvae were present at

the three sites all year round in densities ranging from c21 to 4207 m�3 at Kau Bay,c19

to 381 m�3 at Island Bay and c15 to 220 m�3 at Oteranga Bay. Larval abundance was

Table 1

Asymmetrical ANOVA of differences in larval density among locales, site in locale and months

Effect df SS MS F-value p-value

Locale (harbour vs. coast) 1 5.519 5.519 30.239 b0.001

Site in locale 1 0.641 0.641 3.503 0.0649

Month 8 4.985 0.623 3.405 0.0021

Site�month 16 8.130 0.508 2.784 0.0010

Residual 79 14.418 0.183

All data were log10 transformed. Comparisons were made among nine months; data were not collected for the

month of July; data were incomplete, and thus omitted, for December and March. On occasions where months

were represented in both years, data were included in the analysis from both months.

Fig. 3. Mean number (FS.D.) of settled mussels per pad (0.04 m2) at (a) harbour sites and (b) coastal sites. Means

are standardised to a 30-day month. Note logarithmic y-axes in both figures.


greatest during August 1998 at Kau Bay, in February 1999 at Island Bay and in May 1999

at Oteranga Bay.

Asymmetrical ANOVA revealed significant differences in larval density that were

explained by variation in locale, month and the interaction term of site�month (Table

1), which indicated that larval density changed differently as a function of time among

the three sites (Fig. 2). The term explaining most variation in the data set was locale

(i.e., the contrast between Cook Strait and Wellington Harbour), followed by the

site�month interaction. Site nested in locale (i.e., the contrast between the two Cook

Strait sites) was not significantly different.

Table 2

ANOVA of differences in larval recruitment density among sites and months


Site 3 26.251 8.750 166.478 b0.001

Month 9 47.429 5.270 100.262 b0.001

Month*site 27 13.916 0.515 9.806 b0.001

Residual 68 3.574 0.055

All data were log10 transformed. Comparisons were made among 10 months, data were not collected for the

month of October and were incomplete, and thus omitted, for July.

Table 3

ANOVA of differences in larval recruitment density among pooled sites (harbour vs. coastal) and months


Month 9 48.073 5.341 65.580 b0.001

Site 1 25.021 25.021 307.191 b0.001

Month*site 9 11.475 1.305 16.022 b0.001

Residual 88 7.168 0.081

Comparisons were made among 10 months as data were not collected for the month of October and were

incomplete, and thus omitted, for July.


3.2. Larval densities at recruitment

Recruitment densities at the two coastal sites were less than those at the two

harbour sites. At Matiu-Somes Island and at the Front Lead recruitment maxima of

16,914 mussels per pad (=0.04 m�2) and 2654 mussels per pad were recorded in July

2000 and April 2000, respectively. At Moa Point and Island Bay, maxima of 144 and

297 per pad were recorded in December 2000 and February 2001, respectively.

Numbers of recruiting mussels at Matiu-Somes Island showed small peaks of

abundance in May and September, with a very large peak in July. Data from the

Front Lead revealed a small peak of abundance in April, but unfortunately settlement

pads were lost from this site in July (Fig. 3a). The two coastal sites showed

considerable similarity in the timing of recruitment, with peaks of mussel abundance

being observed in the summer months (Fig. 3b). Significant differences in recruitment

density were explained by site and by month of sampling, as well as by a significant

interaction term that indicates temporal differences in recruitment densities among the

sites (Table 2). ANOVA of pooled data from the two coastal versus the two harbour

sites gave fundamentally the same result as the ANOVA of data from the four separate

sites (Table 3).

4. Discussion

The absence of mussels from rocky intertidal sites on Cook Strait was first noted by

Morton and Miller (1968). Although the intertidal community at such locations is not well

developed when compared with locations in Wellington Harbour, other suspension feeding

invertebrates such as barnacles (Chamaesipho brunnea and C. columna), a number of

different sponge, hydroid and tunicate species and the porcelain crab (Petrolisthes

elongatus) occur at these sites, as do mobile invertebrates such as gastropods, chitons,

decapods and echinoderms. The existence of this intertidal and shallow subtidal

community indicates that the Cook Strait water column can support competent planktonic

larvae of many species and that the substrate at these sites is suitably conditioned to

facilitate successful settlement, metamorphosis and recruitment to adult populations of

most species. A body of evidence now exists which indicates that the primary reason for

the absence of mussels is a low quality particulate food supply (Gardner, 2000; Gardner

and Thompson, 2001), possibly resulting in bottom up community regulation (Helson,


2001). However, this aspect of community regulation is predicated on two important

assumptions. First, that mussel larvae occur in the water column at Cook Strait sites (i.e.,

larvae can be transported to these sites regardless of whether such larvae are physically

able or biologically competent to settle), and second, that competent mussel larvae do

settle at these sites (i.e., the Cook Strait environment is conducive to larval settlement).

4.1. The density and time of occurrence of planktonic larvae

On most sampling occasions when planktonic mussel larvae were collected from the

water column, densities within the harbour were higher than at coastal sites. We therefore

reject the null hypothesis that states that the density of planktonic mussel larvae is similar

at harbour and coastal sites. However, what is not clear from these data is the extent to

which this difference in larval density is limiting recruitment and the subsequent

establishment of an adult population.

Estimates of mussel larval abundance in the water column are very variable in both

time and space (Rao et al., 1975; Dare, 1976; Booth, 1977; Rajagopal et al., 1998a),

and our data are consistent with this variability. In the immediate vicinity of outer

Cook Strait, Wellington Harbour is the only source of mussel larvae, that is, mussels

are almost entirely absent from outer Cook Strait sites on both the North Island and

South Island sides (pers. obs., both authors). The harbour has a rapid flushing time of

only 10 days (Heath, 1971), which means that many, if not all, harbour-derived

mussel larvae with a planktonic duration of 2–4 weeks will be transported out of the

Harbour and into Cook Strait before they are competent to settle at harbour sites.

Once out of Wellington Harbour and into Cook Strait, the larvae will be transported

with the predominant surface flow, the net direction of which is southeastward out of

Cook Strait (Heath, 1971; Bowman et al., 1983a). However, surface flow is highly

irregular, rapid and turbulent, being strongly influenced by local wind conditions, and

to a lesser extent, by tidal currents (Gilmour, 1960; Bowman et al., 1983a), with the

result that some northwestward transport is often possible. Given the duration of the

planktonic larval stage, the fact that mussel larvae occur in the water column all year

round, and the relative proximity of Cook Strait sites to Wellington Harbour, mussel

larvae derived from harbour populations must be widely dispersed through this region.

It is also the case that mussel larvae which are derived from sites outside the region

will be transported into Cook Strait as a consequence of the mixing of three separate

bodies of water at the northwestern approach to Cook Strait (Gilmour, 1960; Heath,

1971; Bowman et al., 1983a, b). The occurrence of major peaks of larval abundance

from spring to late summer at Kau Bay (harbour) and Island Bay (Cook Strait) is

probably attributable in part to the geographic proximity of Island Bay to the large

populations of mussels in the harbour that are the nearest source of larvae. However,

peak larval abundance at Oteranga Bay, which is c18 km west of the harbour, was

observed in the winter, out of phase with the timing of larval supply from harbour

populations. The pronounced difference in the timing of larval peak density between

Island Bay and Oteranga Bay is therefore probably attributable to their difference in

distance from Wellington Harbour as well as the fact that Oteranga Bay receives

water, and therefore larvae, from three distinct water masses. In the context of the


present work, the source of planktonic mussel larvae is unimportant. What is

important is that we have observed larvae in the water column at Cook Strait sites on

every sampling occasion.

Observed densities of planktonic larvae at Kau Bay in Wellington Harbour were 21–

4207 m�3, this was significantly higher than at coastal sites (Island Bay, 19–381 and

Oteranga Bay 15–220 m�3). While larval densities were substantially lower at coastal

sites, they were still high enough to permit successful recruitment at Cook Strait sites (see

subsequent section). It therefore does not seem likely that recruitment limitation in the

form of low larval density prevents mussel settlement and subsequent recruitment at Cook

Strait sites. However, the degree to which it restricts recruit numbers and thus enables

predation or other post-settlement mortality to prevent any further development of an adult

population is unclear and is currently under investigation.

4.2. The successful recruitment of mussel larvae

Recruitment of mussel larvae at all four sites was recorded on every sampling occasion,

and occurred at much higher densities at the two harbour sites than at the two coastal sites.

We therefore reject the null hypothesis that states that mussel recruitment is not

significantly different at harbour and coastal sites.

Numbers of mussels settling onto artificial substrates at harbour sites were high (up

to 422,860 individuals m�2), and between 10 and 20 times higher than at the coastal

sites. The high degree of temporal and spatial variability in these numbers is consistent

with pronounced recruitment variability (orders of magnitude) for a number of different

species, including Mytilus edulis, M. galloprovincialis, Perna perna and P. viridis

(Dare, 1976; Ceccherelli and Rossi, 1984; Petraitis, 1991; Caceres-Martinez et al.,

1993; Lasiak and Barnard, 1995; Molares and Fuentes, 1995; Rajagopal et al., 1998b).

Such variability is attributable to a number of factors, including spatial scale, local

hydrography, substrate type and topography, existing community structure, wave

exposure, water velocity, chemical stimuli and height on the shore (Chipperfield, 1953;

Seed, 1976; Keough and Downes, 1982; Petraitis, 1991; Rajagopal et al., 1998b;

McQuaid and Phillips, 2000). In the present study, mussel recruitment at harbour and

coastal sites occurred throughout the year, consistent with reports from New Zealand

and elsewhere for different mussel species (Ralph and Hurley, 1952; Meredyth-Young

and Jenkins, 1978; Buchanan and Babcock, 1997; Ramirez and Caceres-Martınez,

1999). In addition, the occurrence of successful recruitment at both coastal sites

indicates that mussel larvae occurred in the plankton at these sites were not prevented

from settling (at least onto the artificial substrate used here) by physical or chemical

factors, and were biologically competent to settle at any time during the year. Botsford

et al. (2001) and Gaylord and Gaines (2000) used models to demonstrate that strong

(N4 cm s�1) unidirectional alongshore flow can have dramatic effects by reducing

population persistence in heterogeneous environments. However, they also state

(Gaylord and Gaines, 2000) that temporal variability in the flow field and high pulses

of recruits may reduce the impenetrability of such a barrier. Given that the net

southeastward surface flow in Cook Strait is highly irregular and some northwestward

transport is often possible (Gilmour, 1960; Bowman et al., 1983a), the effect of flow-


induced barriers to successful settlement is likely to be reduced significantly at these

sites. This is further emphasized by the successful settlement of larvae onto the

artificial collectors, but we cannot discount the possibility that successful settlement

onto the natural substrate may be hindered, although this seems unlikely (see next

paragraph).

The primary substratum at all four sites (and indeed at most locations in the harbour and

along Cook Strait shores where one could reasonably expect to find mussels) consists of

the sedimentary rock, greywacke. Observations of mussel populations in Wellington

Harbour confirm that this substrate is rapidly, abundantly and directly settled by mussels

when other biological and physical factors are suitable. We also note that extensive mid-

intertidal bands of mussels are found at most harbour sites, whether they be on greywacke

reef, concrete seawalls or wooden wharf pilings. In short, these mussels have the ability to

settle rapidly on just about any hard substrate and to cover that surface very quickly.

Therefore, suitable substrate type in not likely to be limiting recruitment at Cook Strait

sites.

5. Conclusion

The aim of this study was to determine whether an absence of larval supply and/or an

absence of larval recruitment were factors contributing to the absence of mussels from

Cook Strait sites. Although planktonic larval density and subsequent recruitment were

significantly greater at harbour sites, we observed mussel larvae in the water column and

subsequent recruitment throughout the year at harbour and coastal sites. The authors

further suggest that predation is unlikely to be preventing the establishment of an adult

population given the almost total absence of any mussels, the spatial extent of this absence

(tens of kilometers) and that relatively few predatory species (e.g., the whelks, Neothais

scularis, Lepsiella scobina and Haustrum haustorium, and the star fish Stichaster

australis) are found here (pers. obs., both authors). We therefore conclude that the absence

of mussels at Cook Strait shores is unlikely to be attributed to recruitment limitation, but

rather post-settlement mortality.

Acknowledgements

We thank Nicole Philips for helpful comments on the manuscript, Robert Williamson of

the Island Bay Marine Laboratory for technical assistance and Shirley Pledger and Edith

Hodgen of Victoria University of Wellington for statistical advice. This research was

supported by funding from the School of Biological Sciences and the Island Bay Marine

Laboratory (both of Victoria University of Wellington) to JPAG.

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contrasting patterns of mussel abundance at neighbouring sites: does recruitment limitation explain...

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